1. Field of the Invention
The present invention relates to a Frequency-Temperature-Compensated circuit of Quartz Qrystal Oscillator.
2. Description of the Prior Art
Quartz Qrystal Oscillators are widely used in various field because of its excellent frequency stability. However, the Quartz Qrystal Oscillators for mobile Radio communication systems are required very high frequency stability, miniature size and low consumption.
The frequency stability of the quartz crystal oscillator means the frequency tolerance at room temperature and the frequency stability versus ambient temperature.
In the highly accurate crystal oscillator, following means are essential; a fine frequency adjusting means using a trimmer capacitor and a frequency-temperature compensating means for improving the frequency-temperature characteristic of a quartz crystal resonator in combination with a voltage-dependent electric element such as a variable capacitance diode and a temperature-dependent electric element such as a thermistor. A trimmer capacitor, however, has such drawbacks due to its mechanical structure as; unstability in capacitance especially for a miniaturized type and large size for a high capacitance-stability type. These make realization of a miniaturized highly accurate and stable quartz crystal oscillator very difficult.
To solve the above drawbacks, a floating gate MOS variable capacitor was recently invented and has been introduced to the quartz crystal oscillators. In this capacitor, the capacitance can be varied electrically by injecting electrons into the floating gate, through a thin insulating film, and, hence, the capacitance is kept constant after completing the injection of electrons into the floating gate.
Details of this capacitor has been disclosed in the following papers; Y. Hattori and R. Matsuzaki, "Application of Floating MOS variable Capacitor for the Watch IC.", Proceedings of 11th International Congress of Chronometry, P9 to 12, 1984, and "Miniaturized High-Accuracy Crystal Oscillator With Electrically Adjustable Frequency", J.E.E.,P32 to 36, January, 1986. By using this variable capacitor, a miniaturized high accuracy quartz crystal oscillator with a capability of electrical frequency adjustment was realized. It should be noted, however, that frequency-temperature dependence of the oscillator itself can not be adjusted merely by injecting electrons into the floating gate of this variable capacitor. Therefor, a variable capacitance diode must be added to compensate the frequency-temperature characteristic beside this variable capacitor. This results in increase in the number of components and thus in an increase in size of the oscillator and the production cost.